U.S. Pat. No. 4,674,834 discloses a page scanner employing a fiber optic bundle which is linear at one end and rectangular at the other. The linear end defines a linear entrance field for light corresponding to a scan line or segment astride a page to be scanned. The other end of the bundle is gathered in a pig tail, fused, cut, and polished flat. The polished end defines an (area) exit field for the light emerging from the bundle and is optically coupled to a photosensor array of matching geometry.
The photosensor array, preferably comprises a plurality of discrete sensors accessible on a random access basis such as a dynamic random access memory (DRAM) as described in detail in the above noted patent or a charge injection device (CID) or photodiode array. A charge coupled device (CCD) can be used also but is relatively slow because it is organized on a sequential access basis.
A page scanner of the type disclosed in the above-noted patent employs a look-up table to relate the positions of the pixels in the entrance field with sensors of the sensor array. The look-up table includes the addresses of sensors in the sensor array chosen during an initialization procedure. The addresses so chosen are stored in a sequence to interrogate the sensors, during normal operation, in the proper sequence to organize the pixels exiting the exit face of the bundle to match the organization of those pixels at the entrance face.
The initialization procedure is carried out, for example, by passing a light slit, having a long dimension perpendicular to the axis of the entrance field, along that axis. The slit is moved in increments small compared to the diameter of a fiber. Thus, for a 5100 fiber bundle, 81/2 inches long (600 dots per inch), the slit may be incremented between 12,000 and 20,000 steps. The sensor array includes a larger number of sensors (photosensitive detectors) than the number of fibers in the bundle. Thus, there are typically more than eight sensors for each fiber end. A sensor is selected for each fiber during an initialization procedure by one of several software programs as disclosed, for example, in U.S. Pat. No. 4,762,391 issued Aug. 9, 1988.
The look-up table is stored as an address sequence or string in a read only memory (ROM, typically a programmable ROM) and is read out from ROM each time the linear entrance field is moved down a page to each of the consecutive scan lines or segments of the page. That is to say, the page is moved, with respect to the linear field, to a scan segment and the scan segment is exposed to light. The reflected light enters the fibers, travels in the fiber, and impinges on the sensors. Consequently, the address string is used as pointers to access and organize the pixel data exiting the data field via the sensor array to correspond to the ordered sequence of fibers in the entrance field. The paper then is moved to the next scan segment and the operation repeats until the entire page is scanned.
It is to be understood that the fiber optic bundle is illustratively non-coherent. That is there is no known relationship between the positions of the ends of the fibers in the entrance field with the positions of the ends of the respective fibers in the exit field. Further, it is to be understood that the initialization procedure is carried out once during manufacture of the scanner and produces a ROM which becomes part of the finished product and is unique (has a unique address string) for each scanner manufactured. The initialization procedure can be carried out expeditiously and is expected, in mass production, to be accomplished in an average time on the order of seconds.
A problem which arises when a DRAM sensor array such as the Micron optical DRAM is used to sense a pattern of pixels is related to the fact that the DRAM does not operate as a true random access array when exposed to light. That is to say, when a DRAM is used as a light-sensitive array, it cannot be operated so that a single selected sensor can be exposed, read out, and refreshed for a subsequent cycle without also exposing and refreshing other sensors in the array. Because of the non-random operation, the sensors selected by the address string may be exposed to different exposure (soak-read cycle) times during normal operation of the scanner.
The exposure of the sensors to different soak-read cycle times is clearly demonstrated by an example. Let us assume that the exit field of a fiber optic bundle of an illustrative scanner is in a fixed position, either attached to or coupled via a lens, with respect to a random access type photosensor array (e.g. DRAM, CID) and the address string is determined and stored in ROM. It is clear that one can read the proper sequence of sensors may from left to right as one might view the sequence of fiber ends across the linear entrance field of the bundle. Let us also assume a realistic soak-read cycle time of 500 nanoseconds. For a 5100 fiber array, when the last sensor of the address string is read, its value represents a soak-read cycle time of over 2.55 milliseconds (5100.times.500 nanoseconds) whereas, the first sensor has a soak-read cycle time of 500 nanoseconds.
Another problem which arises for both DRAM and CID (charge injection device) area sensor arrays is one of smearing. Smearing occurs when the page being scanned is illuminated continuously rather than illuminated by pulsed light synchronized with the movement of the page.
Specifically, in operation, a cycle commences with the movement of the entrance face of the fiber bundle to a scan segment across the page--here assuming continuous illumination. Each sensor is reset, and precharged, as it is read. The sensor then integrates the subsequent light flux until it is read again, one cycle time later. The cycle time equals the time needed to step the page so that the next scan segment can be read. The step to the next scan segment occurs when the instant cycle is completed, just as the next cycle is about to begin.
Under such operating conditions, the charge accumulated on the photodetector corresponding to the first fiber in the entrance face represents the first pixel of the previous scan segment; the charge from the last fiber represents the last pixel of the instant scan segment. The charges accumulated from all other detectors is a mixture, in varying proportions, of the pixels of the previous and the instant scan segments. The worst case is at the center of the scan segment in which proportions are 50/50. Thus, at the center of a scan segment, the resolution is half that at the edges.
In color scanning, using time sequential color illumination, as is the case for many of the present generation of scanners, the colors are likewise mixed when an area sensor array accessible on a random access basis is used. The percentages of two colors that are mixed depends on when the colors are mixed in relation to reading instance. In any case, two of the color signals are always mixed in varying proportions across a scan segment.